There are three general mechanisms by which an object can release heat energy: conduction, convection and radiation. In conduction, heat is transferred within a substrate. In convection, a hot region of a gas or liquid moves away from the source of heat and is replaced by cool media. In radiation, energy is emitted as photons or electromagnetic waves without a medium.
Heat dissipation is a significant concern for electronic devices. Poor thermal management can reduce the efficiency and performance of electronic devices. For example, overheating may cause displays to be dimmer and processors to operate more slowly. Extended overheating can impair reliability, cause device malfunctions and may ultimately result in premature device failure. Heat generation may also cause discomfort to users of portable electronic devices because the heat generated within the device can raise the surface temperature and injure the user's skin.
Conduction and convection have been extensively used for thermal management of electronic devices. Heat sinks are well-known devices that provide conductive cooling. Convective cooling is typically achieved with mechanical fans. Conduction and convection may be paired together, such as by including heat sinks and mechanical fans within the tower of a desktop computer.
Conduction and convection are often not suitable for small electronic devices due to the sizes of the conductive and convective cooling devices. Devices such as heat sinks and mechanical fans occupy a large proportion of the available space in electronic devices and are difficult to physically integrate into smaller items such as cellular phones, LEDs and tablet computers.
A film that dissipates heat of a surface to which it is applied by enhanced radiative cooling, referred to as a “molecular fan”, has been previously described in U.S. Pat. Nos. 7,931,969 and 8,545,933 as a heat dissipation device for use in electronic devices. A molecular fan is a coating that may be applied to a surface to increase substrate surface emissivity and thus to enhance “active” heat dissipation by radiation. The molecular fan takes advantage of the high emissivity in the infra-red of discrete molecules (as opposed to extended solids) which result from transitions between different vibrational states. The molecular fan includes particles to increase surface area, and materials on the surface of the coating that will radiate infra-red light as they transition between different vibrational states. An emulsion that hardens upon curing is also included in a molecular fan coating material, to adhere the particles and other materials on the surface being coated. The molecular fan coating provides good surface hardness, provides resistance to fingerprints, inhibits corrosion and is easy to clean.
Thermal management becomes even more difficult in environments where air is unable to freely circulate, thus inhibiting convection. Confined spaces produce a pocket of air, referred to as an “air gap”, which acts as an insulator and traps heat. The tendency of manufacturers to produce smaller and thinner electronic devices concentrates the heat generated by the internal components and promotes the formation of these insulating air gaps. Insulating air gaps can also be formed in locations where electronic devices are mounted.
Various cooling techniques that have been used in small electronic devices include heat pipes, phase change materials (PCMs), Peltier devices, microscale ionic “winds” and heat spreaders (sheets or films of substances with relatively high thermal conductivity such as metals and graphite). These cooling techniques were found to provide insufficient heat dissipation, and to be too expensive or complicated to manufacture for commercial use.